Waimea is perfect for catching rainbows because the East windward side is often in blowing mist and the West leeward side is often filled sunny skys. Any time of year can be good with tradewinds and sunny leeward side. Best time, though the rarer of the 2 is morning, because you are looking West with back to the wind/rain. Quite often, the afternoon will produce a rainbow but you are facing into the rain. I often turn my back on the wind and rainbow, set all camera settings, wipe lens with chamois and quickly turn and fire hand held, then turn back and repeat. Rainbows are lower to horizon from about 3hours before sunset (or after sunrise) and climb higher and full 1/2 circle as sun approaches setting/rising. Exposure is easy, just watch out your shadow doesn't intrude into the picture.